The invention relates generally to apparatus for measuring mechanical accelerations, and more specifically to a single coil force-balance velocity geophone.
A conventional geophone has a single coil-mass, and a magnet, both of which are contained in a housing. Generally, springs connected to the housing support the coil, thus allowing motion in one dimension relative to the housing, and the magnet is fixed with respect to the housing. The relative motion of the coil, in the magnetic field of the magnet, induces an electromotive force ("emf"), or voltage, in the coil. Accordingly, geophones may also be constructed in which the magnet is attached movably in the housing while the position of the coil is fixed with respect to the housing.
The coil is often a solenoid of finite length, in which a copper wire is wound circumferentially around a hollow, cylindrical, bobbin-form. By "single-coil," it is meant that a single, continuous length of wire has been wound into a coil, and that connection to the coil is available only at the two ends of the wire. Specifically, in a single-coil design, there is no access to electrical signals at any intermediate point within the coil. This is in contrast to dual-coil and multiple-coil transducers, in which there are separate connection points (terminals) at both ends of each coil in the transducer.
In many conventional single-coil velocity geophones, the coil may actually be split into two electrically-connected, but physically-displaced, coils, which have been wound in opposite directions. However, in these geophones, the displaced coils are still formed from one length of wire, and more importantly, connection to displaced coils is made only at the ends of the wire. That is, even in these "split-coil" geophones, one has access to electrical signals only at the outer ends of the coils. There is no capability for accessing points between the split-coils. In short, the conventional single-coil velocity geophone is inherently a two-terminal device, regardless of whether it utilizes split-coils.
FIG. 1 of U.S. Pat. No. 5,119,345 the disclosure of which is incorporated by reference, provides a cutaway drawing of a typical, conventional, single-coil, velocity geophone. The voltage across these two terminals provide the output signal from the device. This voltage is proportional to the velocity of the housing, for frequencies above the natural resonant frequency of the spring-coil-mass system. The voltage falls off as the square of frequency, for frequencies below this resonant frequency. Important performance characteristics for geophones are their bandwidth, linearity, and dynamic range.
The bandwidth of the geophone, as a velocity sensor, is normally expressed as the frequency range over which the output response (typically in volts per unit case velocity) of the sensor is nearly constant. In this instance, the term "constant" typically implies that the response does not vary by more than a factor of two. In a conventional geophone, this frequency range normally extends from just above the resonant frequency to a frequency that is typically twenty times the resonant frequency. As an acceleration sensor, however, the conventional geophone, has a peaked response at its natural frequency, and a much smaller bandwidth. This is shown in FIG. 1. In FIG. 1, the signal output from a conventional geophone is shown on the y-axis 100, as frequency is varied along the x-axis 105, in response to both a constant velocity 115 and a constant acceleration 120. The vertical line 110 depicts the natural resonant frequency of the conventional geophone. In most applications, a conventional geophone is used to record ground velocity signals at frequencies above its natural resonant frequency, because the response does not vary over frequency.
The linearity is often expressed in terms of total harmonic distortion (THD). THD is defined as the ratio of the root sum of squares of output voltages arising at integer multiples of a particular driving frequency to the voltage at that driving frequency, when the geophone is driven with a monotonic velocity. Higher linearity devices exhibit lower THD values. Linearity is limited typically by the magnetic non-linearities associated with the motion of the coil in a non-uniform magnetic field gradient, and by the mechanical non-linearities associated with large deflections of the spring which supports the coil.
The dynamic range is normally expressed as the ratio of maximum measurable signal to the minimum resolvable signal, in a given bandwidth of frequency (e.g., 1 Hz). The minimum resolvable signal is limited typically by electronic noise in the recording system (due partially to the source impedance represented by the coil). The maximum measurable signal is limited by the maximum allowable displacement of the coil-mass (d.sub.max, typically 1 mm, peak-to-peak). This displacement limit leads directly to a full-scale velocity (equal to 2.pi.fd.sub.max, at frequency f) or full-scale acceleration (equal to (2.pi.f).sup.2 d.sub.max, at frequency f) input range for the conventional geophone. The full-scale velocity or acceleration range of any one conventional geophone is fixed by the mechanical design of its components, and cannot be changed in a simple manner.
Conventional single-coil velocity geophone technology is highly evolved. The sensors are compact, rugged, and inexpensive. They are manufactured in large volume, with a low unit cost, by three major manufacturers: Geo Space Corporation (Houston, Tex.) Mark Products (Houston, Tex.) and Sensor NL (Netherlands). A wide range of resonant frequencies are available (1 Hz to .about.400 Hz).
Examples of conventional geophones are the Mark Products Ultraphone, UM-2, Sensor NL SM-4-LD, and the Geo Space GS-30CT. Conventional geophones have typical large signal THD values of .ltoreq.0.1%. Conventional geophones can exhibit dynamic ranges in excess of 10.sup.6 (100 Hz bandwidth). Higher bandwidth, linearity, and dynamic range are normally attained in conventional geophones through more careful design, assembly, and test. In general, conventional geophones with higher performance specifications, have a higher unit cost.
Many applications require direct sensing of acceleration, rather than velocity. Specifically, seismic techniques for imaging the subsurface structure of the earth, can generate improved data if the sensors measure acceleration directly. This is because of the fact that typically, the "signal" of interest (longitudinal or shear bulk waves), is at higher frequency than certain unwanted "background" signals (e.g., surface propagating Rayleigh waves). As the acceleration amplitude is equal to the velocity amplitude times 2.pi.f, a measurement of acceleration amplitude will provide a higher ratio of "signal" to "background" than will a measurement of velocity. Unfortunately, conventional geophones have reduced bandwidth when measuring accelerations, compared to their performance for velocity measurement.
In addition, new, state-of-the-art applications in seismic imaging are requiring further improvements in linearity, over the levels provided by conventional geophones. Finally, many applications require accelerometers with user-selectable measurement ranges. All of these applications are highly constrained in terms of cost.